Re: AMD Bulldozer topology regression since 4.6

Borislav Petkov
2016-11-29T21:15:23Z

On Tue, Nov 29, 2016 at 10:02:00PM +0100, Brice Goglin wrote:
> However thread_siblings isn't back to where it was in 4.5. Now we have a
> single bit in each thread_siblings mask. That's correct with respect to
> the sysfs topology documentation. In 4.5, there were two bits (one for
> each core of the compute unit), which was wrong (cores with different
> core_ids shouldn't appear in each other thread_siblings). I assumed that
> these processors had to break the sysfs topology documentation to expose
> the concept of "dual-core compute-unit" which somehow sits between
> hyperthreading and dual-core.

This is exactly the problem - there's never proper fitting of the
compute unit "ideology" in the whole topology view. So what we're doing
now is revert to the old strategy of keeping Bulldozer have core_id ==
thread_id. Basically, all CU threads are cores, as they're more powerful
than, say, SMT threads, but they still share an FPU.

So it is a different kind of sharing and it's kinda, well, different.
There's no other way to put it.

> I personally do not care much about this regression, not sure about
> other user-space tools?
>
> Another minor related change: /proc/cpuinfo shows "cpu cores : 16"
> instead of "8".

Can you send me the whole thing? Offlist is fine too.

Thanks.